LONDON (Reuters) - British shale gas company Cuadrilla has started fracking for natural gas in northwest England, seven years after the practice was halted in Britain for causing earth tremors.

Environmental groups strongly oppose the practice of hydraulic fracturing, or fracking, which involves extracting gas from rocks by breaking them up with water and chemicals at high pressure.

But the government, keen to cut Britain's reliance on imports as North Sea gas supplies dry up, has tightened regulations and has given Cuadrilla permission to frack two wells at its Preston New Road site in Lancashire.

Cuadrilla, 47.4 percent owned by Australia's AJ Lucas and 45.2 percent owned by a fund managed by Riverstone, said it would spend at least three months fracking two horizontal wells to test flow rates to determine whether full-scale gas extraction would be viable.

The British Geological Survey estimates shale gas resources in northern England alone could amount to 1,300 trillion cubic feet (tcf) of gas, 10 percent of which could meet the Britain's demand for almost 40 years.

"Shale gas has the potential to be a new domestic energy source, enhancing our energy security and delivering economic benefits, including the creation of well-paid, quality jobs," the Department for Business, Energy and Industrial Strategy said.

"We have been clear that any shale developments must be safe and environmentally sound," the department added in a statement.

Attempts to extract the gas have come under fire from local communities and campaigners concerned about the potential effect on the environment and ground water. They say extracting more fossil fuel is at odds with Britain's commitment to reduce greenhouse gas emissions.

"It is morally bankrupt to be heralding the start of a whole new fossil fuel industry," Craig Bennett, chief executive of environmental group Friends of the Earth said in a statement.

"You can deal with climate change or you can have fracking – you can’t do both," he said.

The government asked its climate advisers on Monday to examine whether Britain should toughen its climate targets and set a date to move to a net zero emissions goal.

Britain has an existing target to cut its greenhouse gas emissions by 80 percent compared with 1990 levels by 2050.